1. Field of the Invention
The present invention relates to a field of displacement measurement for optically measuring displacement information such as a displacement amount and a displacement velocity of an object.
2. Related Background Art
Encoders have been conventionally used to optically measure displacement amounts and displacement velocities of objects. FIG. 11 shows a typical arrangement of a conventional optical encoder. This encoder includes a light source 11 for generating a light beam, a lens 12 for collimating the light beam, a movable scale 13 having grating slits and a stationary scale 14 having grating slits having the same pitch as that of the slits of the movable scale 13. Note that these grating slits have a pitch which does not substantially induce a diffraction effect. The encoder also includes a photodetector 15 for detecting amounts of light passing through the movable and stationary scales 13 and 14, and a signal processing circuit 16. With this arrangement, when the scale 13 moves, the light amount detected by the photodetector 15 is periodically changed in correspondence to scale movement. More specifically, when the phases of the two slits coincide with each other, viewing them from the light source side, a maximum light amount is detected. Also, when the phases of the two slits of the scales 13 and 14 are shifted by 180.degree., a minimum light amount is detected. When the movable scale 13 is moved by one pitch of the slits, a period signal having one period is obtained. In the signal processing circuit 16, this period signal is converted into signal pulses, and these pulses are counted to measure a displacement of the movable scale.
In the conventional encoder of the type using these two scales, i.e., the movable and stationary scales, the positions of the two scales must be aligned with each other, and the position of each scale must be aligned with the detection system. In order to increase the detection resolution, high assembly precision is required to result in high cost. To increase the detection resolution is to decrease the pitch of the slits. However, when the pitch is excessively decreased, diffracted light components are produced to degrade displacement measurement. For this reason, the increase in detection resolution in this encoder is limited.